Polymer-Matrix Composites (PMC's) are extremely useful for many applications due to their high performance, strength and stiffness combined with their low weight and resistance to corrosion. However, despite the benefits of PMC's, they are very sensitive to elevated temperatures. Even brief exposure to elevated temperature can result in significant irreversible degradation of key physical properties of such fiber-reinforced PMC's which are commonly used.
Apparently, a key mechanism in heat-related damage of PMC's is chemical rather than physical degradation of the resin component of the composite. Thus, significant losses of strength, on the order of 10-50%, can occur with no macroscopic indicators such as internal or surface delamination, cracking, swelling or color change. Since there are no macroscopic indicators, it is impossible to detect such changes using traditional techniques such a x-ray imaging or ultrasound, which function to image macroscopic or microscopic physical features. The physical features which would represent a loss of structural integrity in other commonly used construction material are simply not present; except in severely damaged PMC parts.
However, due to the characteristics of non-damaged PMC's, their use where exceptional strength and lightness are required is rapidly expanding. PMC's are used in critical structural components for the aerospace industry, weapons manufacturing, transportation, construction and a variety of other fields. In these areas, even a minor degradation of structural strength is a serious concern as the safety of large numbers of people and very expensive machinery depends on the structural members used. Additionally, since many PMC items are made as large pieces, they can be very difficult and expensive to examine with techniques associated with the examination of smaller parts or samples of parts.
Further, although relatively expensive and difficult imaging techniques may be utilized to detect some of the damage in PMC's, such techniques are limited to large companies in limited applications due to their prohibitive cost. These techniques are only employed in industries such as the aerospace industry where the potential liability for the failure of a damaged PMC structural member can be astronomical. Smaller companies making less critical uses of PMC's cannot afford the high cost of presently available non-destructive testing techniques, since the costs are not offset by a corresponding risk. Analysis of PMC components for items such as oars, bicycles, kayaks, canoes, surf boards, etc. must be relatively simple and inexpensive before such analysis will be routinely used for quality control/quality assurance programs in these fields.
It is therefore an object of the present invention to provide a method for the optical identification of heat damage in PMC parts.
It is a further object of the present invention to provide a low cost method for optically detecting heat damage in PMC's.
It is yet a further object of the present invention to provide a method for imaging damage in PMC's which allows for the rapid evaluation of large parts.
It is a further object of the present invention to provide a high speed method for imaging damage in PMC's.
it is still a further object of the present invention to provide an apparatus for the evaluation of heat damage in PMC parts utilizing Laser Induced Fluorescence (LIF).